1. Field of the Invention
This invention generally relates to improvements in laser optical systems and, more particularly, to improvements to the optical cavities of flash lamp pumped dye lasers.
2. Description of the Prior Art
The flash lamp pumped dye laser is a type of laser capable of tunable emission throughout most of the visible electromagnetic spectrum. A dye laser consists of a lasing medium, a means of exciting or optically pumping the medium and an optical cavity. The lasing medium can be any one of a plurality of fluorescent organic dyes such as rhodamine 6G or Na-fluorescein. The exitation source can be either another laser or a broad band flash lamp. The optical cavity consists of two opposed mirrors that continuously reflect the radiation emitted by the lasing medium back into itself. The two mirrors retain the radiation within the optical cavity so that the energy from the lasing medium can accumulate.
The process of lasing occuring within the lasing medium, hereinafter referred to as the dye, begins when the molecules of dye are excited from the lowest levels of the ground singlet state S.sub.0 to higher vibrational-rotational levels of a second singlet state S.sub.1 by absorbing light from either a second laser or a flash lamp. Thereafter the molecular energy of the dye molecules decays nonradiatively to the lower energy levels of the excited singlet state S.sub.1. Laser emission occurs during the stimulated transition from the bottom vibrational band of the excited singlet state S.sub.1 to one of the lower energy levels in the ground singlet state S.sub.0.
A general description of dye lasers and fluorescent organic dyes is given in the article entitled "Flash Lamp-Excited Organic Dye Lasers" by Mr. B. Snavely, in the proceedings of the IEEE, Volume 57, No. 8, August 1969 at page 1374.
In the past flash lamps used in dye lasers have had short operating lifetimes and required frequent replacement. Usually these flash lamps were cooled with water and exploded in the first minutes of operation under the shock of high power pumping. Also, these flash lamps often developed cracks in their glass envelopes and became unusable after a short period of service. In addition, the glass envelopes often became discolored and opaque from electrode sputter and the output from the flash lamps rapidly degraded.
A further problem experienced with dye lasers has been the short operating lifetime of the fluorescent dyes. When organic dye molecules are pumped in an optical cavity using a broad band flash lamp, the radiation emitted from the flash lamp is in the infrared, visible and ultraviolet portions of the electromagnetic spectrum. The ultraviolet radiation has high energy and, heretofore, was very destructive to the dye molecules. Exposure to ultraviolet light changes the chemical structure of the molecules through the process of photolization and the molecules thereafter no longer fluoresce. This destruction of the dye required that dye be periodically replaced.
In addition, the conventional flash lamp operates with a very high black body temperature and produces a significant amount of infrared radiation. This infrared radiation was, heretofore, absorbed as heat by the dye and caused thermal distortion in the optical path through the pumping cavity. The heat resulted in changes in the refractive index of the dye solution and produced a poor quality output beam.
These problems are further described in an article entitled "Study of a One Watt Repetitive Dye Laser" by Mr. C. Loth and Mr. Y. Meyer, in Applied Optics, Volume 12, No. 1, January 1973 at page 123.